1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus for driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for reducing a memory size in data modulation and preventing deterioration in picture quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal to thereby display a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a moving picture. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):τr∝γd2/Δε|Va2−VF2|  (1)where τr represents a rising time when a voltage is applied to a liquid crystal, Va is an applied voltage, VF represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of liquid crystal cells, and γ represents a rotational viscosity of the liquid crystal molecules.τf∝γd2/K  (2)where τf represents a falling time at which a liquid crystal is returned into an initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an elastic constant.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon, a moving picture is blurred out on the screen.
Referring to FIG. 1, the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in the data by using a look-up table, (hereinafter referred to as high-speed driving scheme). This high-speed driving scheme allows data to be modulated by a principle as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. In the high-speed driving scheme, |Va2−VF2| is increased from the above equation (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame period. Accordingly, the LCD employing such a high-speed driving scheme compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon from a moving picture, thereby displaying a picture at desired color and brightness.
In other words, when there is a change upon the comparison of the most significant bit data MSB of the previous frame Fn−1 and the most significant bit data MSB of the current frame Fn, the high-speed driving scheme selects the modulated data Mdata corresponding to the look-up table and modulates as shown in FIG. 3. Such a high-speed driving scheme only modulates a few high order bits for reducing the load of the memory size upon implementation of hardware. The high-speed driving scheme implemented in this way is shown in FIG. 4.
Referring to FIG. 4, a conventional high-speed driving apparatus includes a frame memory 43 connected to a most significant (or high-order) bit bus line 42 and a look-up table 44 connected to the most significant bit bus line 42 and the frame memory 43.
The frame memory 43 stores most significant bit data MSB during one frame period and supplies the stored data to the look-up table 44. Herein, the most significant bit data MSB are high-order 4 bits in source data RGB Data In having 8 bits.
The look-up table 44 compares the most significant bit data of the current frame Fn inputted from the most significant bit bus line 42, with the most significant bit data of the previous frame Fn−1 inputted from the frame memory 43 in Table 1 or Table 2, and selects the corresponding modulated data Mdata. The modulated data Mdata are added to the least significant (or low-order) bit data from a least significant bit bus line, and supplied to a liquid crystal display.
TABLE 101234567891011121314150013467910111214151515151510124579101112131415151515201235789101213141515151530123568910111214141515154001246791011121314151515500023578911121314151515600013468910111314151515700012457810111214141515800012356891112131415159000123467910121314151510000012457810111314151511000002356791112141515120000013457810121315151300000123468101113141514000000123579111314151500000001246911131415
In Table 1, a left column is for a data voltage VDn−1 of the previous frame Fn−1 while an uppermost row is for a data voltage VDn of the current frame Fn.
Thus, in the high-speed driving scheme which only modulates 4 bits of the most significant bit data MSB, a data width of the frame memory 43 and the look-up table 44 is 4 bits.
But, if the data width of the look-up table 44 is limited to the number of the bits of the most significant bit data MSB, the modulated data value registered at the look-up table 44 is limited accordingly. For example, if the modulated data value of a high gray level does not have a desirable value and is limited to lower than that, a picture quality is deteriorated because the brightness desired can be obtained in the high gray level.
To reduce such a deterioration and to modulate the data in a desirable way, a data width of the modulated data registered at the look-up table 44 should be large enough and the inputted source data should be compared by full bits (8 bits). It is inevitable to increase the memory size of the look-up table 44 for this purpose. That is, if the full bits (8 bits) data is inputted to the look-up table 44 from each of the previous frame Fn−1 and the current frame Fn and the modulated data registered at the look-up table 44 is set to the full bits (8 bits), the memory size of the look-up table 44 become 65536×8=524,000 bits. Herein, in the foregoing equation, the first term ‘65536’ is a multiplication (256×256) of each full bit source data of the previous frame Fn−1 and the current frame Fn, the second term ‘8’ is the data width (8 bits) of the modulated data registered at the look-up table 44.